I tried the following commands:
1. create tablespace TEST_WITHOUT_PATH datafile 'TEST_WITHOUT_PATH.dbf' size 1M;
or 2. alter tablespace TEST add datafile 'TEST_WITHOUT_PATH.dbf' size 1M;
All commands execute correct, and the TEST_WITHOUT_PATH.dbf in ORACLE_HOME/database/
My instance parameter DB_CREATE_FILE_DEST is empty.
And in the book 'OCA/OCP ORACLE Database 11g', it is said 'OMF can always be overridden by specifying a datafile name in the CREATE TABLESPACE command.'
So it should be nothing with the DB_CREATE_FILE_DEST, since I specify the datafile name here.
I searched the internet, but failed to find the answer, so I asked here. Thanks.